The primary research question of the proposed study is this: Is a water-based exercise plus cognitive training intervention efficacious in improving cognitive outcomes for Veterans with Mild Cognitive Impairment (MCI)? We address the need to improve cognitive function in this vulnerable population through a novel combined training program that will combine two interventions: water-based exercise and cognitive training. The proposed four-year randomized controlled trial will compare water-based exercise + cognitive training (WATER+CT) to a Usual Care Control Condition (UC). The primary aim of the proposed research is to evaluate the efficacy of WATER+CT to improve cognitive function in Veterans with MCI. Secondary aims of the proposed research are: 1) to evaluate the efficacy of WATER+CT to improve peak oxygen consumption and 2) to evaluate the durability of the cognitive benefits derived from WATER+CT. Exploratory aims of the proposed research are to evaluate moderators and mediators of treatment response. These aims will be tested in a single-blind randomized controlled trial that will compare the efficacy of WATER+CT to UC. This trial will include 190 Veterans meeting criteria for MCI age 50-90 with half randomized to WATER+CT and half randomized to UC. For the WATER+CT condition, the exercise training (i.e., WATER) consists of a six-month long individualized exercise program of water-based exercises. During this phase, Veterans will come to thrice-weekly group sessions at Aquatic Therapy Center at the VA Palo Alto Health Care System (VAPAHCS). After completion of the exercise program, Veterans will begin classroom-based cognitive training at the VAPAHCS for up to two months. The CT is based on an efficacious training program that is structured around two components, pre-training and mnemonic training, both of which have been used successfully in persons with MCI. Veterans randomized to the UC control condition will receive educational materials about brain health in addition to their usual care, which is the care they would typically receive in the VA. After completion of the active treatment phase, Veterans will transition to the six-month long follow-up phase. At the end of the six-month long follow-up phase, participants will return to the VA for a final follow-up visit. Participants will complete a variety of neuropsychological measures taping into areas of cognition such as attention, executive functioning, and memory. Participants will also undergo physical fitness assessments including a 6-minute walk test and an exercise treadmill test. To study possible predictors of response to treatment, we will also collect biological (cardiovascular functioning and BDNF plasma levels) and genetic data (APOE and BDNF genotypes) from these participants. We hope to provide evidence of the efficacy of an exercise training augmentation for cognitive training thus laying the groundwork for future VA health care system wide deployment of a lifestyle intervention to prevent Alzheimer?s disease.